Capital refused to comment on the speculation which drove Prudential's shares up to 567p before they slid to 540p, 1% lower on the day, after markets were rattled by the downgrading of Greece and Portugal's debt by credit rating agencies.

Prudential, which refused to comment, needs to secure the support of 75% of its investors for the takeover at a shareholder meeting at the end of May, after launching its rights issue on 5 May. The insurer is still attempting to persuade them to back the transaction, which will transform its business into one dominated by the fast-growing economies of Asia.

Eamonn Flanagan at Shore Capital was not surprised at the speculation and expected it to continue as the deal proceeded. Flanagan said: "The hurdle rate for the deal, at 75% of votes cast, is a high one and, in our view, the opposition to the deal suggests it could be a close call. We reiterate our buy recommendation, with the shares offering significant upside potential if the deal does not proceed."

One major shareholder expressed doubt about the transaction, on which chief executive Tidjane Thiam is staking his reputation.

"There is a very good chance they won't get the 75% needed, in which case management would be in a very difficult position – effectively a vote of no-confidence in the strategy of the company," the shareholder told Reuters. "There is merit in the argument that more value might be realised if Pru was broken up. We have encouraged Pru to sell UK business and focus on the Asian business – it's an argument certainly worth exploring."

Among the other companies said to have been approached by Capital were Resolution, which refused to comment, as did Aviva.

• This article was amended on 28 April 2010. The photo caption was corrected to read that Tidjane Thiam is Prudential's chief executive.